Using metals in sketchfab
"Metal" textures are often determined by a number of different textures working together to create the appearance of metal. These can include a number of things such as Diffuse, Specular, Normals, and glossy shaders. In order to make these usable by external programs you can either bake these maps individually of create a PBR material, which is basically a combination of all of the above.

There are lot's of tutorials on how you may create PBR materials in blender my favorite as always is from Blender Guru.

However if you wish to export individual maps there is a great tutorial here on 
stack exchange.